A wireless transceiver is widely and generally used. In a mobile phone which uses the wireless transceiver, a UMTS (W-CDMA) system with using a CDMA (Code Division Multiple Access) and a GSM with using a TDMA (Time Division Multiple Access) system as a multiple access system are main systems.
In a UMTS/GSM multimode PA module which supports both of the UMTS and GSM, suppression of gain variation among products has been a critical problem.
As a technique of stabilizing a gain of a conventional PA module, a technique of adjusting a value of an external resistor in accordance with process variation is cited.
As a prior art for compensating the variation in the manufacture, a technology disclosed in Japanese Patent Application Laid-Open Publication No. 2001-237656 (Patent Document 1) can be cited.
Further, “A Practical Method of Designing Analogue Electronic Circuit” (Non-Patent Document 1), page 132, FIG. 6.1 discloses a technique of adjusting an amount of a current flowing through a bias circuit by a current mirror circuit.
FIG. 1 is a circuit diagram showing the configuration of the bias circuit described in this Non-Patent Document 1.
The current mirror circuit of this diagram is a circuit controlled to flow a current defined by a ratio of a threshold voltage of a FET with respect to a constant current source “IREF”. In this case, the following formula is obtained.IREF=β×(VG+Vth)2  [Formula 1]
An above-described relation is established. Here, “Vth” is a threshold voltage of a MOSFET configuring the current mirror circuit. Further, “β” represents a coefficient described later. This formula is modified as follows.
                              V                      G            ⁢                                                  ⁢            1                          =                                                            I                REF                            β                                +                      V            tl                                              [                  Formula          ⁢                                          ⁢          2                ]            
An above-described relation is established. When a transconductance (current/voltage conversion gain) “gm” of an FET transistor “To” is obtained from these relations, the following formula is obtained.
                                                        gm              =                            ⁢                              2                ×                β                ×                                  (                                                            V                                              G                        ⁢                                                                                                  ⁢                        1                                                              -                                          V                      th                                                        )                                                                                                        =                            ⁢                              2                ×                                  β                                ×                                                      (                                          I                      REF                                        )                                                        1                    2                                                                                                          [                  Formula          ⁢                                          ⁢          3                ]            
Here, “n” represents a gate-width ratio of a reference FET with respect to the FET transistor To.